This disclosure relates to the determination of the state-of-health (SOH) of a battery. Specifically, the invention relates to systems and methods for determining the SOH of a rechargeable battery and, in particular, a lithium battery. The invention, however, is not limited to lithium batteries but is applicable to any rechargeable battery that degrades over time, thus necessitating the determination of the battery's SOH.
Over time, rechargeable batteries age and degrade causing a decrease in the batteries' ability to hold a charge and to deliver its rated current to a load. Eventually, a battery will need to be replaced after it has degraded to the point that it no longer effectively holds a charge. The SOH of a battery indicates the returnable (usable) or net capacity of a battery cell through its cumulative stress-life. The SOH of a battery is also an indication of how closely the battery meets its design specifications.
Rechargeable batteries are used in many different fields where it is critical that the battery have a minimum capacity following charge so that the battery may perform its intended function. Capacity is the maximum charge that a battery is capable of holding. The capacity of a brand new battery should be approximately that which is indicated by the manufacturer. Determining the minimum capacity of a battery is of particular importance in applications such as medical devices, military weapons, and aircraft emergency power applications where failure of the battery due to insufficient charge may result in disastrous consequences.
The monitoring of the battery's state-of-charge (SOC) is desirable in rechargeable battery cells. The SOC is the quantification (in percentage (%)) of the usability of the cell in terms of its relative charge level. A traditional SOC indication for a lithium cell is generally related to the terminal voltage of the cell during a stabilized open-circuit stand. The open-circuit voltage (OCV) functions much like a fuel gauge to convey when the cell is fully charged, fully depleted, or at some other usable state between those two end points. However, as discussed above, the cell will age and degrade over time such that the degradation mechanism causes the cell's impedance to increase. During charge, an aged cell will reach a specified end-of-charge voltage (EOCV) and correctly report 100% SOC, as the correct indication that the cell has received its maximum available charge. As the battery ages, however, even though the battery indicated 100% SOC at the completion of charging, after some settling time has elapsed, the subsequent OCV response for an aged cell will decline and indicate SOC of less than 100% due to the rise in the cell's impedance. Thus, it has been found that there is a need for a separate measure from the SOC “charge status” fuel gauge that will directly capture the returnable or net capacity of the cell through its cumulative stress-life.
There are a number of approaches that have been posited for determining the SOH of a battery. For example, a first approach calculates the SOH by measuring the internal resistance of a battery. When a battery is experiencing high internal resistance, this is an indication that the SOH of the battery is low (i.e., poor). However, this measurement alone does not provide a true estimation of the battery's SOH. Specifically, this estimation disregards various factors (i.e., the decrease in internal resistance over time as well as the effect of temperature and protective circuits) that affect the SOH of the battery. While this estimation is good for laboratory uses or infrequent stationary uses, it is generally unable to be applied directly to an application.
Additionally, the type of battery to be measured causes a number of problems when trying to determine the SOH of a battery using the above method. Different rechargeable batteries behave in different manners, have different battery chemistries, and are available in different types from different manufacturers. Some common examples of rechargeable batteries include lithium, lithium ion, lithium nickel, nickel metal hydride, nickel cadmium, and lead-acid.
Another approach to determining the SOH of a battery is the full/partial discharge test. In this approach, the battery is either fully or partially discharged by subjecting the battery to a constant load. During the discharge time, the battery voltage is monitored such that the time it takes for the battery to drop to a certain voltage is compared with that of a healthy battery. This comparison allows for the calculation of the SOH of the battery. However, there a number of drawbacks associated with this approach. This method is expensive, time consuming, and requires the battery to be off-line during testing.
Yet another approach to determining the SOH of a battery involves the use of stand-alone battery monitoring systems. During the aging of a battery, these systems measure the value of one or more electrochemical parameter(s) of the battery. The SOH of the battery is then determined based on the way the parameter(s) changes over time. However, because a history of measurements of the parameter(s) must accumulate before the degradation of these parameter(s) can be determined, the stand-alone battery monitoring systems cannot determine the SOH of the battery without first acquiring these measurements over time.
It is desirable to provide a system for measuring the SOH of a battery which ameliorates the disadvantages discussed above. There is a need for determining the absolute minimum capacity required to perform the intended function of the battery after the battery has been charged in various applications, as discussed above, without the need to perform a running computation of the SOH.